A train whistling at constant frequency is moving towards a station at a constant speed V. The train goes past a stationary observer on the station. The frequency `n^(')` of the sound as heard by the observer is plotted as a function of time t, figure. Identify the expected curve.

Options

A.
B.
C.
D.

Answer

C

Explanation

Let the original frequency of the source is `n_(o)`.
Let the speed of sound wave in the medium is v .
As observer is stationary

Apparent frequency `n_(a)=((v)/(v-v_(s)))n_(o)` (when train is approching)
`=((v)/(v-v_(s)))n_(o)=n_(a)gtn_(o)`
When the train is going away from the observer
Apparent frequency `n_(a)=|((v)/(v+v_(s)))n_(o)=n_(a)ltn_(o)`
Hence, the expected curve is (c).